Recently, a thin film transistor (TFT) using an oxide semiconductor as a channel layer, a so-called oxide TFT has been widely used. The oxide TFT has characteristics of having high mobility with respect to the existing amorphous silicon (a-Si) TFT. For this reason, in a display device, the oxide TFT is adopted as a driving circuit, and thus, a reduction in the size of the circuit and a miniaturization of the wiring can be realized.
In the display device, a reduction in the size of the driving circuit and a miniaturization of the wiring contribute to the improvement in an aperture ratio of a pixel. As a result thereof, high resolution of the display device can be realized.
In a case where the oxide TFT is used as a peripheral driving circuit of a display adopting a liquid crystal display (LCD) or an organic light emitting diode (OLED), such as a pixel switch and a vertical shift register (VSR), excellent initial properties and long term stability are required.
For such a request, a driving method is proposed in which in a double gate type oxide TFT in which a top gate electrode is added to a bottom gate type oxide TFT, a negative potential is applied to the top gate electrode (for example, Japanese Patent Application Laid-Open No. 2012-19206). The negative potential is applied to the top gate electrode, and thus, a threshold voltage of the oxide TFT is moved to a positive side. As a result thereof, normally off of the oxide TFT is realized, and thus, excellent initial properties are obtained.